This invention relates to filter apparatus useful for removing fine (submicron) particulate material from a stream of air or other gas, the particulate material being in solid or liquid form. More particularly, the invention relates to filter apparatus of the type in which pollutant particles in a gas stream are given an electrical charge and the charged stream is then passed through a granular filter bed in which the bed granules are polarized by the application of an electric field to the bed. The polarized granules attract and retain the charged particulate material so that it is separated from the gas stream. An electrical charge may, for example, be applied to the particulate material prior to passage through the filter bed, by passing the polluted stream through an ionizer, where it is subjected to a high voltage corona discharge. Electrification of the bed granules may be achieved by producing an electric field between a high voltage electrode immersed in the bed and a grounded bed-retaining structure.
Filter apparatus of the above type may for convenience be referred to as electrified filter bed apparatus, and its technology is well developed. The apparatus exhibits advantages compared with other forms of electrical filtering apparatus; for example, due to the use of a granular filter bed, the apparatus can provide a large collection surface area in a small volume. (Filtering efficiency is related to the available collection area.) Further, in the subject apparatus, the collection surface may be an inexpensive durable material, such as gravel, which gives the apparatus cost advantages.
Copending U.S. patent application Ser. No. 69,046 filed Aug. 23, 1979, U.S. Pat. No. 4,308,036, the subject matter of which is incorporated herein by reference, discloses electrified filter bed apparatus of the above type. The prior application relates, inter alia, to the concept of causing the granular bed to move downwardly as a rigidified plug during passage of polluted gas therethrough by applying an electric field to the bed of suitably high intensity. This is found to improve the filtering efficiency by trapping the particulate material in the bed granules. The prior application also discloses other concepts relating to the design and operation of electrified filter bed apparatus.